The invention disclosed in this application relates to digitizer tablets and tablet structures have a working surface which may be illuminated. For example, the digitizer tablet may be a back-lighted or "surface-lighted" digitizer tablet, or the digitizer tablet may be translucent and back lighted by an external light source such as a light box, or the digitizer tablet structure may form part of a back-lighted or "surface-lighted" tablet, etc.
Digitizer tablets and their use are well known. See, for example, BYTE magazine, January, 1989, pages 162-174. In certain applications, illumination of the working surface is highly desirable. This is particularly true of applications that require accurate tracing of drawings, prints, photographic images such as radiology, etc., and CAD applications. Frequently such applications require a large tablet, e.g., one having a 36.times.48 inch working surface.
An illuminated digitizer working surface may be provided in a number of ways. For example, digitizer tablet structure including a grid or equivalent structure which functions as part of the position determining portion of the digitizer may be back lighted. A back-lightable, translucent digitizer tablet (including grid structure) is disclosed in U.S. Pat. No. 4,206,314 (Prugh et al.). A back-lighted digitizer device is disclosed in co-pending U.S. patent application Ser. No. 07/370,913, of Siefer and Purcell, filed June 23, 1989, which is assigned to the assignee of this application. The back-lighted digitizer device disclosed in the '913 patent application includes a translucent digitizer tablet supported over a lighting compartment in which are mounted fluorescent lamps for back lighting the digitizer tablet.
The working surface of a translucent and back-lighted digitizer tablet or tablet structure may show undesirable lighting effects such as patterns, lines or spots which may detract from or interfere with use of the digitizer. One reason is that the light which illuminates the working surface is projected through the entire digitizer tablet or tablet structure including the grid or equivalent structure, which, in the case of electromagnetic digitizers, includes a shield spaced from grid wires of the grid structure. Another reason, as discovered by the applicants herein, is that structure causing such undesirable lighting effects projects images which are focussed at or near the illuminated working surface.
The Prugh et al. '314 Patent discloses neither the problem of undesirable lighting effects at the illuminated working surface of a back-lighted or translucent digitizer tablet, nor a solution.
The Siefer and Purcell '913 application discloses a digitizer tablet structure which eliminates many undesirable lighting effects of a back-lighted digitizer device. Specifically, the '913 application discloses an electromagnetic digitizer tablet comprising a sandwich tablet structure which diffuses light projected to, and/or defocuses any imaging of the electromagnetic grid structure on, the top working surface of the digitizer tablet, so that any such imaging is barely, if at all, noticeable at a typical user's distance of about 18 inches. Included in the sandwich structure is a clear or transparent spacer disposed between the grid structure and the working surface of a translucent top element.
A novel digitizer tablet having a working surface illuminated by light projected through the working surface but not through the entire tablet or tablet structure is disclosed in co-pending U.S. patent application Ser. No. 07/350,042, of Siefer, filed May 10, 1989, which is also owned by the assignee of this application. The tablet disclosed in the Siefer '042 application, which may be referred to as a "surface-lighted" digitizer tablet, includes a light source disposed between the position-determining structure and the top working surface. In that surface-lighted digitizer tablet, the light which illuminates the top working surface does not pass through the position-determining structure. Accordingly, the position determining structure does not cause the undesirable lighting effects described above for a back-lighted digitizer tablet to appear at the working surface. The surface-lighted tablet of the Siefer '042 application includes a clear or transparent spacer between the light source and a translucent top element, which diffuses light from the light source to assist in providing an even distribution of light on the working surface.
However, even in the back-lighted digitizer device disclosed in the Siefer and Purcell '913 application and the surface-lighted digitizer tablet disclosed in the Siefer '042 application, undesirable lighting effects such as uneven illumination of the tablet's working surface may occur. Furthermore, moisture or liquids used during manufacture or use may accumulate between the clear or transparent spacer and the top translucent element, which may produce further undesirable lighting effects, such as dark, bubble-shaped patterns, from ambient light reflected from the surface, and bright bubble-shaped patterns from light emitted by the light source through the working surface. Also, pressure applied to the working surface from a stylus, cursor or a writing instrument or from a user leaning on the working surface, from heavy objects placed on the working surface, or from the weight of the tablet elements themselves, may cause temporary or permanent local deformation of elements of the digitizer tablet, which may also cause undesirable lighting effects.
There is thus a need to improve the illumination of the working surface of digitizer tablets and like structures.